The present invention relates to helical antennas for portable wireless devices and, more particularly, to a helical antenna including a magnetic tune for selectively receiving electrical waves of a particular frequency and a capacitance adjuster for reducing any mismatching between the characteristic impedance of an associated antenna feeder and the input impedance of the antenna resultant from varying the inductance of the antenna coil with the magnetic tuner.
Selective reception of a particular frequency among several discrete frequencies is often required for small portable wireless devices. Thus, the antenna or input circuit of a wireless device, for example, is preferably designed so that its inductance can be varied to tune the device into an electromagnetic wave of particular frequency.
In an attempt to meet this demand, conventional small portable wireless devices have been provided with a helical antenna whose inductance may be varied. Such a helical antenna is shown, for example, in FIG. 6. This antenna includes a hollow tube 1' with an antenna element 2' wound thereabout so as to form a coil. The tube 1' has an antenna connector 3' at the bottom end thereof and an antenna cap 4' at its upper end. The antenna further includes a frequency-tuning ferrite magnet 5' which is vertically movably fixed to the inside of the hollow tube 1'. As is apparent, with this arrangement the inductance of the coil can be varied by varying the vertical displacement of the magnet. The tuning ferrite magnet can be moved vertically and will stay at a selected position within the antenna coil. This enables, apparently, the selective reception by the wireless device of an electromagnetic wave of particular frequency.
It has been found, however, that the input impedance of the antenna will vary as the ferrite magnet is moved to vary the inductance of the coil and therefore the input impedance will no longer match the characteristic impedance of the associated feeder which connects the antenna to the wireless device. This resultant mismatching will make it practically impossible for the wireless device to receive the electromagnetic wave selected by tuning the antenna.
Furthermore, the vertical movement of the ferrite magnet in the antenna coil of this prior art device does not cause a substantial variation of inductance because the hollow tube is large in diameter so as to ensure that it is resistive to strong winds and other external forces. More particularly, because the ferrite magnet of this prior art device moves in a large space defined by the hollow tube, there is no appreciable change in inductance even though the ferrite magnet can be moved a relatively long distance along the longitudinal axis of the tube. Accordingly, it has been found that practically no fine tuning is possible with this antenna.